A magnetic random access memory (MRAM) is a solid-state non-volatile magnetic storage device. Bits of data are stored in small magneto-resistive elements. For example, in a magnetic tunnel junction (MTJ) magneto-resistive element, two ferromagnetic layers, a pinned magnetic layer and a sense magnetic layer, are separated by an insulating tunnel barrier. Magnetoresistance results from the spin-polarized tunneling of conduction electrons between the ferromagnetic layers. The tunneling current depends on the relative orientation of the magnetic moments of the two-ferromagnetic layers.
The magnetization orientation of the sense layer is used for information storage. The resistance is either low or high, depending on the relative orientation of sense magnetic layer with respect to the pinned magnetic layer. The orientation is either parallel (P) or antiparallel (AP).
In an MRAM array, orthogonal lines pass under and over the magneto-resistive elements, carrying current that produces switching fields. The magneto-resistive elements are designed so that the magnetization of the sense magnetic layer will not switch when current is applied to just one line, but will switch when current is applied to both lines. A magneto-resistive element is manufactured using series of layers of material stacked on top of one another. A pinned layer can be either on the bottom or top of the tunnel junction layer. The sense layer is required to be patterned in two dimensions in order to define a discrete bit. When the pinned layer is on the bottom, the pinned layer can be defined in two dimensions (i.e., is xe2x80x9cpatternedxe2x80x9d) or can be defined along one dimension (i.e., is xe2x80x9cunpatternedxe2x80x9d). What is meant by xe2x80x9cunpatternedxe2x80x9d is that the pinned layer is defined in only one dimension and not defined in the other dimension so that the resulting structure is a line. When the pinned layer is patterned, it is typically patterned at the point in the manufacturing process when the sense layer is patterned. When the pinned layer is to remain unpatterned, the sense layer is patterned and the patterning process is controlled so that the pinned layer is not patterned by the sense layer patterning process.
If the pinned layer is patterned, it contributes to the magnetic state of the sense layer. This magnetic contribution is often referred to as the xe2x80x9cdemagnetization fieldxe2x80x9d. This demagnetization field needs to either be eliminated or tightly controlled.
The pinned layer is generally composed of a set of several material layers that have some lattice mismatch. Strain relaxation and the resultant columnar grain growth creates morphological roughness. This roughness contributes to the magnetic state of the sense layer. This contribution is sometimes referred to as the xe2x80x9ccoupling fieldxe2x80x9d or xe2x80x9cNe""el couplingxe2x80x9d. The coupling field also needs to be either eliminated or tightly controlled.
One solution to reduce the coupling field is to put the sense layer on the bottom of the stack, thereby reducing the Ne""el coupling due to the roughness of the pinned stack. In this case, the pinned layer is patterned in order to perform patterning of the sense layer. As discussed above, the patterned pinned layer will then contribute to the demagnetization field.
Additionally, when the pinned layer is patterned, the sense layer of the completed magneto-resistive element will switch polarity using different mechanisms. When the sense layer switches from P to AP (low resistance to high resistance state switching), the predominant mechanism is the growth of end domains. When the sense layer switches from AP to P (high resistance to low resistance state switching), the mechanism used is the growth of a domain from somewhere in the middle of the sense layer. The AP to P switching has an unstable nucleation mechanism due to probabilistic switching nucleated by surface irregularities and/or roughness. This unpredictable nucleation mechanism in a patterned pinned layer junction causes variations in the switching field over many bits and also for multiple cycling of the same bit.
A magneto-resistive element with an unpatterned pinned layer always switches by the same mechanism. Thus, the switching of a magneto-resistive element that has a patterned pinned layer has an increased switching distribution relative to a magneto-resistive element that has an unpatterned pinned layer.
In accordance with the preferred embodiment of the present invention a magneto-resistive element is constructed. A ferromagnetic sense layer is deposited on a surface. The ferromagnetic sense layer is patterned. An ion etch is performed in preparation for depositing a dielectric layer. The dielectric layer is deposited over the sense layer. A ferromagnetic pinned layer is deposited over the dielectric layer.